a/ \(y'=\frac{\left(3x+1\right)'}{cos^2\left(3x+1\right)}=\frac{3}{cos^2\left(3x+1\right)}\)
b/ \(y'=2cos\left(3x^4-5\right).\left[cos\left(3x^4-5\right)\right]'=-24x^3.cos\left(3x^4-5\right).sin\left(3x^4-5\right)\)
\(=-12x^3.sin\left(6x^4-10\right)\)